Cutting inserts, which provide cutting edges of a cutting tool, are made from a material with high abrasion resistance such as tungsten carbide. Such cutting inserts are rigidly secured to a tool body by means of screws so that they can perform stable cutting during a cutting process. FIG. 1 is a perspective view of a milling cutter, to which a prior art cutting insert is applied. A screw is perpendicularly fastened to a cutting insert seating surface of a cutter body, wherein a bottom surface of the cutting insert is brought into contact, thereby securing the cutting insert to the cutter body. In such a case, since the screw exerts only a perpendicular fastening force to the cutting insert, the screw fails to sufficiently support the reaction force corresponding to a cutting force and can be unfastened during operation of the milling cutter. Accordingly, there is a problem in that the cutting insert is not rigidly secured to the cutter body.
FIG. 2 is a perspective view of another prior art milling cutter, wherein a screw-fastening structure is modified in order to solve the above-mentioned problem. In this structure, a screw is fastened to a cutting insert seating surface of a cutter body and is slanted at a predetermined angle. With the slantingly fastened screw, the fastening force of the screw is allowed to have both perpendicular and horizontal components. Thus, the screw can be maintained as fastened, despite the reaction force corresponding to the cutting force, and the cutting insert can be more stably secured to the cutter body. However, even with such screw-fastening structure, there is a problem in that when subjected to a cutting environment accompanied by heavier vibrations, the screw cannot withstand such an environment and thus becomes unfastened.